Lead-acid batteries have been used for many diverse applications. Such applications include use as a starting, lighting and ignition power source for vehicles (SLI), use in marine batteries for starting, lighting and other auxiliary power requirements, as a motive power source for use in golf carts and other vehicles and other applications of this sort. In addition, lead-acid batteries have been employed in a variety of stand-by power applications to provide a power source when the main power source becomes inoperable, as by, for example, interruption of electricity. Lead-acid batteries have also been employed in many other applications, e.g., uniform power distribution, power damping applications, and even for small electronic devices such as video cameras and cassette players.
While the extent of discharge and the particular cycling requirements of a lead-acid battery for a specific application vary widely, one criterion remains constant: it is important to ensure that proper charging of such batteries is carried out. Thus, on the one hand, undercharging such lead-acid batteries can result in less than optimum output and service life. Undercharging can result in perhaps permanent sulfation of part of the active materials, as well as stratification of the electrolyte and uneven use of the active materials.
On the other hand, undue overcharging of lead-acid batteries likewise creates problems. Overcharging of lead-acid batteries thus can result in permanent damage of the batteries as well as presenting potential safety hazards caused by, for example, boiling the electrolyte of the battery. Further, overcharging lead-acid batteries can accelerate positive grid corrosion and even lead to bulging and/or buckling of the battery plates. Among other undesirable aspects of undue overcharging are the undesirable increase in the specific gravity of the electrolyte, possible oxidation of the separators and the undue heat generated that can accelerate various problems.
The time and manner in which lead-acid batteries are charged is also important for other reasons. Thus, many applications require charging within a relatively short period of time so that it is important to optimize the current or voltage used, while, at the same time, avoiding the use of excessively high currents that will result in gassing and the like.
Further, the design of a suitable charger for lead-acid batteries that will allow optimization of the charging procedure is extremely complex. Typically, the state-of-charge (or state-of-discharge) of the battery requiring charging is not known. Also unknown are such factors as the specific use history of the particular battery, as well as the age and maintenance history, all of which can affect the optimum charging requirements. Similarly, the internal temperature of the battery is either unknown, or, if known, the ability to compensate for the particular internal temperature in the charging process is quite difficult.
Another important factor complicating battery charger design is the type of battery being charged. Commercially used lead-acid batteries thus represent a broad spectrum of widely varying designs, ranging from flooded-type batteries (both maintenance-free and batteries requiring maintenance during service life) to valve-regulated recombinant sealed lead-acid batteries (where essentially all of the electrolyte is retained in the plates and separators and charging gas is recombined to water within the battery).
The size of the battery or cell being charged must also be taken into account. The requirements for charging a 12-volt battery are different from those requirements for a 6-volt battery or for a single lead-acid cell.
Additional factors that must be taken into account in determining the charging regime include the rate at which the battery was discharged and the stand time since discharge. The composition of the battery grids will affect the charging regime as will the presence of electrochemical impurities.
Another important factor that must be considered is the safety aspects of a battery charger. There is the potentiality for fumes, fire, explosion or thermal runaway when a battery such as an internally shorted battery or a sealed valve regulated recombinant battery is over-charged with typical fixed voltage chargers.
Still further, a stand-alone battery charger configuration may allow design considerations that could not be tolerated in a vehicle charging apparatus. Thus, in a vehicle, the dynamics of the battery utilization must be considered, viz., the charging regime may be taking place alternatively while the battery could be called on to deliver power due to a myriad of conditions.
For these and other reasons, it is not surprising that there has been considerable effort over a period of many years to provide battery chargers suitable for lead-acid batteries that overcome one or more of the problems involved. Thus, U.S. Pat. No. 3,421,067 to Wilson et al. discloses a battery charger control circuit which includes a coulometer to accurately measure the state-of-charge of the battery. That coulometer measurement is then used to cause the battery to be charged at the maximum available current rate until a charge exactly equal to the previous discharge has been accomplished.
U.S. Pat. No. 3,424,969 to Barry shows a battery charge control which includes sensing the rate of rise of voltage of the battery while fast-charging the battery. This fast-charging is terminated upon sensing a rate of rise that exceeds a predetermined rate.
U.S. Pat. No. 3,517,293 to Burkett et al. discloses charging a battery by imposing an increasing charge on the battery by charging during certain intervals and by providing discharge intervals interspersed with the charge intervals.
U.S. Pat. No. 3,816,806 to Mas involves discharging the battery periodically during the charging process.
U.S. Pat. No. 3,816,807 to Taylor notes that, in attempts to overcome the effects of temperature and age, efforts have turned to using the gas evolution rate as a more direct indication of the charge acceptance. It is stated that such systems appear to be unstable and have battery aging problems. Taylor discloses a charging sequence using the battery impedance as a charge control parameter.
U.S. Pat. No. 4,629,965 to Fallon et al. shows a charger for a battery which includes an initial charge carried out at a maximum rate of current which tapers in magnitude until the battery attains a certain voltage. Thereafter, the battery is charged with a continuous reduction of current until termination.
U.S. Pat. No. 4,742,920 to Sutphin et al. discloses a microprocessor-directed battery charger which utilizes a dV/dt sensing. A timed finishing charge is used to enable a quicker and more efficient charging.
U.S. Pat. No. 4,829,225 to Podrazhansky et al. discloses a method and a device for rapidly charging a battery by providing a charge pulse to the battery, followed immediately by a depolarization pulse created by allowing the battery to discharge across a load and followed by a stabilization period. The sequence is cyclically repeated until the battery is charged.
U.S. Pat. No. 4,956,597 to Heavey et al. discloses a battery control circuit that first monitors the battery charge during a charge cycle until the charge voltage approaches the voltage range where the gassing point of the battery is anticipated to occur. As the charging voltage reaches the first voltage threshold level, a pulsed loading circuit is activated which periodically places a load across the battery and accurately measures the true output voltage. When the measured pulsed load voltage exceeds the threshold voltage predetermined to be indicative of the entry of the battery into the gassing phase, a timing network is activated and charging is continued for only a predetermined time.
U.S. Pat. No. 5,049,804 to Hutchings discloses what is considered to be a universal battery charger. This charger includes a microprocessor which receives inputs from current, voltage and temperature sensors for controlling the battery-charging profile.
It has further been proposed to use a microprocessor-based device to test the status of a battery using the battery starting characteristics. The battery status monitor determines the battery characteristics from two sets of data. One set of data is collected during normal usage of the battery, while the second set of data is taken during a specialized test profile typically in conditions like highway driving.
The test cycle involves imposing linearly increasing current or voltage ramp onto the battery terminals and then measuring the corresponding voltage or current response of the battery. The presence of a maximum in the dV/DI versus I.sub.ramp or a minimum in the dI/dV versus the V.sub.ramp indicates the gas point.
It was also found that the current at which the gas point occurs in the ramp-up direction is proportional to the battery capacity. The current at which the gas point occurs in the ramp-down direction reaches the lowest possible value when the battery is fully charged. Thus, the battery capacity can be determined from the former, and the state-of-charge from the latter parameter.
Despite all of the considerable effort evident in this field, there still exists the need for a method and apparatus that allows lead-acid batteries to be efficiently recharged. It is accordingly an object of the present invention to provide a method and apparatus for charging a lead-acid battery which is interactive with the battery being charged so the charging will inherently take into account, and compensate for, such factors as the service life history of the specific battery being charged so as to allow the charging profile to be capable of being optimized for that specific battery.
Another object of this invention is to provide a method and apparatus for recognizing the optimized charging current and voltage to achieve charging at a very high efficiency, while minimizing the amount of gassing.
A further object lies in the provision of a battery charger which inherently compensates for the internal temperature of the battery.
Yet another and more specific object of the present invention is to provide a method and apparatus for recognizing the state-of-charge of the battery so as to allow control of the end charge of the battery.
A still further object provides a method and apparatus for compensating for battery flaws such as imbalance from one cell to another as well as internal shorts and the like.
An additional object is to provide a battery charger that is capable of efficiently charging any size of cell or battery.
Other objects and advantages of the present invention will become apparent as the following description proceeds.